Does Blender have a histogram?
Yes, press T in the image viewer to reveal it, among other things.
In addition, clicking on the image shows some useful information about the pixel under the cursor (in particular, the scene referred and display referred RGB values. Plus some fun things such as screen-space XYZ coordinates, where Z is depth).
You will have to render first to use these features, but you can use low-resolution, low sample test renders for fine-tuning.



That said, you might also be interested in utilizing the wide dynamic range of the images cycles spits out to avoid so many small lighting changes.

See 
How to put a Linear Workflow in practice with Cycles and
Render with a wider dynamic range in cycles to produce photorealistic looking images